In the hair domain, it is possible to distinguish two types of dyeing.
The first is the semipermanent or temporary dyeing, or direct dyeing, which involves dyes capable of bringing the natural colour of the hair a more or less marked colour modification which is resistant, where appropriate, to several shampooings. These dyes are called direct dyes; they can be used with or without oxidizing agent. In the presence of oxidizing agent, the aim is to obtain a lightening dyeing. Lightening dyeing is performed by applying to the hair the fresh mixture of a direct dye and of an oxidizing agent and makes it possible in particular to obtain, by lightening of the melanine of the hair, an advantageous effect such as a uniform colour in the case of grey hair or to make the colour stand out in the case of naturally pigmented hair.
The second is permanent dyeing or oxidation dyeing. The latter is performed with so-called “oxidation” dyes comprising oxidation dye precursors and couplers. The oxidation dye precursors, commonly called “oxidation bases” are compounds which are initially colourless or faintly coloured which develop their dyeing power inside the hair in the presence of oxidizing agents added at the time of use, leading to the formation of coloured and dyeing compounds. The formation of these coloured and dyeing compounds results either from an oxidative condensation of the “oxidation bases” with themselves, or an oxidative condensation of the “oxidation bases” with colour modifying compounds commonly called “couplers” and generally present in the dyeing compositions used in oxidation dyeing.
To vary the shades obtained with the said oxidation dyes, or to increase their shimmer, direct dyes are sometimes added to them.
Among the cationic direct dyes available in the field of dyeing of keratinous fibres, especially human keratinous fibres, compounds are already known whose structure is developed in the text which follows; nevertheless, these dyes lead to colours which exhibit characteristics which are still inadequate from the point of view of the intensity and homogeneity of the colour distributed along the fibre; it is said, in this case, that the colour is too selective, and from the point of view of fastness, in terms of resistance to various attacks to which the hair may be subjected (light, adverse weather conditions, shampooings).